Converted Wave Velocity Analysis Based On Scanning Method

One vital key of seismic exploration data processing is the velocity analysis ofconverted wave. It will directly affect the precision of the calculated stack profile later.Due to the asymmetry of converted wave propagation paths, the position ofconversion point is no longer common reflection point position. Therefore, we cannotuse the binning method for converted wave data processing. Different fromconventional hyperbolic, converted wave time-distance curve is no longer elliptic, soall that are suitable for the hyperbolic approximation method cannot be used toconverted wave processing, that is to say, p-wave velocity analysis method is nolonger completely suitable for converted wave velocity analysis.Currently, the P wave velocity analysis methods have become more sophisticated.This paper compares four criterions which are the average amplitude energy,similarity coefficient, non-normalized cross-correlation and statistical normalizedcross-correlation. The first two criterions belong to the same category criterion for theoverlay class, calculate the amount is small, but the average amplitude energy aboutthe trace’s polarity has a high polarity requirement. The latter two criterions belong tothe same category, for the relevant category, relatively large amount of calculation,non-normalized cross-correlation to highlight the strong reflections, normalizedcross-correlation statistics can strengthen the weak reflected signal. Comparing thesensitivity of these methods, cross-correlation has relatively high sensitivity, but italso has its own defects, cross-correlation method is large in amplitude change, due tovariation in the cross-correlation function of the instability, for background with nose,it is very difficult to identify great value.In this paper, the author analyzes and summarizes some key problems inconverted wave velocity analysis on the basis of existing research, summarizes allkinds of calculation method of conversion point position, compares the advantagesand disadvantages of precise solution, iteration method and approximate solution interms of total point gathers extraction, summarizes the calculating methods of thetime-distance curve for converted wave, and compares the precision of Taylor seriesexpansion method, the converted wave square root method and fraction method. We combine the precision solution of conversion point position and double square rootnon hyperbolic time-distance equation method for converted wave velocity analysis.Finally, we get a more accurate velocity information of converted wave thanconventional velocity analysis method.Finally, the author studied a new method of velocity analysis for converted wave,the method can perform quickly velocity analysis for converted wave. It scans themoveout between P wave and converted wave (the common source point gather canbe obtained from the moveout), then using cross-correlation algorithm to identify theaccurate waveform and the accurate velocity of converted wave can be obtained. Themethod has a higher precision and the precision of velocity analysis for convertedwave can be further improved by narrowing step-length of scanning.